carbonite hardness

I have Discus, silver dollar, swordtail, kuhli, betta, ram, goldfish, pleco, blackghost knifefish, Green knifefish, angelfish. The questions is which fish need soft/hard water and how do I softened or hardened water? My GH is 3 dGH KH is 3 dKH pH is 7.5.

I have a tank that is CONSTANTLY/always dealing with low kH issues. It's the water in my area. I'm worried about pH shock for a new, juvenile tankmate next time I have to do a substantial water change. Trying to decide if I should multi-task the function of a new decoration I must acquire for...